Bipolar transistors are key components in high output power density circuits that operate at microwave frequencies. These circuits commonly employ heterojunction bipolar transistors (HBTs) and are increasingly being utilized in large signal applications, such as power amplifiers, oscillators, mixers, modulators, high speed circuits and the like. These large signal applications can create high power densities within the HBT that can lead to significant temperature increases. Although temperature sensitivity is significant for all types of power transistors, it is particularly important for the HBT when fabricated in processes having relatively poor thermal conductivity and a strong dependence of junction behavior on temperature, such as Gallium Arsenide (GaAs)-based processes, Indium Phosphate (InP)-based processes and Gallium Nitride GaAs and the like.
When an HBT operates with high current densities, typically greater than 10 kiloamps per square centimeter (kA/cm2), a commonly observed phenomenon referred to as the self-heating effect occurs. The self-heating effect is characterized by a decreasing current gain with increasing output voltage. The mechanisms responsible for the self-heating effect are generally attributed to a variation in current gain with junction temperature and are the same as those giving rise to the variation of gain with ambient temperature. Thus, as the output voltage of the HBT increases (typically the voltage between the collector and the emitter VCE), the self-heating effect causes the current gain to decrease. This phenomenon is depicted in FIG. 1A, which is a graph of output current density versus output voltage for a conventional HBT transistor in a common emitter configuration. An exemplary load line region 100 for the HBT is also shown. Here, it can be seen that the output current density decreases, or rolls off, as the output voltage increases, generally with greater severity as the base current increases. This, in turn, decreases the gain (β) and leads to DC biasing point and DC quiescent point destabilization on the load line 100 for large signal designs. A graph of output current density versus output voltage for an ideal HBT transistor is depicted in FIG. 1B. Here, the output current density remains constant as the output voltage increases, resulting in a stable DC quiescent point.
One reason HBTs are commonly used is because they have highly efficient power operation at microwave frequencies. However, most applications, such as power amplifier (PA) applications in a communication system, need to have both high efficiency and high linearity. For instance, PA nonlinearity leads to intermodulation distortion and can raise the bit error rate (BER) and, accordingly, is one of the key issues in microwave communication systems. As a consequence, the linearity of the circuit is a major factor in large signal circuit design.
To date, efforts to alleviate the self-heating effect can be classified as semiconductor processing techniques or circuit design topology techniques, both of which have distinct disadvantages. The processing techniques typically increase the processing complexity and lead to lower yield, while the circuit techniques typically result in significant increases in chip area and design complexity.
Thus, improved systems and methods that compensate for the self-heating effect of HBTs and/or improve HBT linearity are needed.